1. Field of the Invention
This invention relates to an internal combustion engine, and more particularly to an improved technique directed to cooling of a four-cycle air-cooled internal combustion engine for a motorcycle.
2. Description of the Background Art
Driving of two camshafts by a crankshaft in an internal combustion engine of the dual over head cam (DOHC) type is well known and is performed using a timing chain or a belt. The same timing chain or belt extends between and around sprocket wheels provided on the two camshafts such that the two camshafts are simultaneously driven at an equal speed. According to the driving system for the camshafts, the chain or belt passes through a chain chamber between cylinders disposed in parallel to each other in a cylinder element (refer to, for example, Japanese Patent No. 2,593,674 (page 1, FIG. 1)).
Further, a driving system for camshafts by a crankshaft in an air-cooled internal combustion engine of the DOHC type is known. In the known driving system, two camshafts are disposed on each of the left and right of a cylinder head, and the left and right camshafts are driven by camshaft driving apparatus disposed on the opposite left and right sides of the engine, respectively. Driving of the two camshafts on each of the left and right is performed such that one of the camshafts is driven by the crankshaft through a chain while the other camshaft is driven by the one camshaft through another chain. According to the camshaft driving mechanism, the camshaft driving apparatus are disposed on the opposite left and right sides of the engine thereby to substantially eliminate the necessity for provision of a chain chamber between the cylinders (refer to, for example, Japanese Patent Publication No. Hei 5-55686 (pages 1 to 2, FIGS. 1 and 2)).
The camshaft driving system of the invention disclosed in Japanese Patent No. 2,593,674 mentioned above is shown in FIGS. 13-a and 13-b. In the camshaft driving system, a space for a timing chain 016 for driving camshafts is defined by a cylinder wall. That is, a chain chamber 031 is provided at a central portion 030 in the longitudinal direction of a cylinder element 0E0, and the timing chain 016 is driven to circulate in the chain chamber 031. The timing chain 016 extends between a sprocket wheel 010h attached to a crankshaft 010 and sprocket wheels 033c, 034c of an equal diameter attached to two camshafts 033, 034, respectively, such that driving rotation of the crankshaft 010 is transmitted to the two camshafts 033, 034 through the single timing chain 016.
Incidentally, this camshaft driving system is adopted by a four-cycle water-cooled internal combustion engine. Thus, where the layout is applied to a four-cycle air-cooled internal combustion engine, the radiation area, for example, for cooling fins or the like cannot be sufficiently achieved from the available space around the cylinders positioned in the proximity of the chain chamber. Also, it is recognized that a corrective measure for handling heat radiation cannot be taken readily. Accordingly, heat cannot be radiated sufficiently from the cylinders which are positioned in the proximity of the chain chamber when compared with the cylinders which are not positioned in the proximity of the chain chamber. Therefore, an non-uniform thermal influence occurs with the cylinder section.
Additionally, the camshaft driving system of the invention disclosed in Japanese Patent Publication No. Hei 5-55686 (pages 1 to 2, FIGS. 1 and 2) mentioned hereinabove is shown in FIGS. 14-a and 14-b. In the known camshaft driving system, two camshafts 033, 034 are disposed on each of the left and right of a cylinder head 03, and driving apparatus for the camshafts 033, 034 are individually disposed on the opposite left and right sides of an engine 0E. A timing chain 016 extends between a sprocket wheel 033c of a large diameter attached to one of the two camshafts 033, 034 on each of the left and right and a sprocket wheel 010h of a crankshaft 010, and the camshaft 033 is driven by the timing chain 016. An inter camshaft driving chain 036 extends between a sprocket wheel 033d of a small diameter attached to the camshaft 033 and a sprocket wheel 034d attached to the other camshaft 034 and having a diameter equal to that of the sprocket wheel 033d of the small diameter, and the camshaft 034 is driven through the inter camshaft driving chain 036.
According to the camshaft driving system, the camshafts disposed two by two on the left and right of the cylinder head are driven by the camshaft driving apparatus disposed on the opposite left and right sides of the engine. Therefore, it is not necessary to provide a chain chamber for a camshaft driving chain between cylinders disposed in a juxtaposed relationship to each other. However, two series of driving apparatus for driving the camshafts are required, and the associated complication of the engine structure and increased number of parts cannot be avoided. Consequently, the cost of the engine is increased.
In such situations as described above, development of an improved structure of an air-cooled internal combustion engine is desirable. The present invention provides such an improved engine, wherein the cooling at a cylinder section is achieved efficiently by a comparatively simple alteration in structure which does not invite complication of the engine structure. Such improved engine includes a camshaft driving system wherein a camshaft driving mechanism for transmitting rotational driving force of a crankshaft to camshafts is disposed between cylinder bores of a cylinder block, and the rotational driving force of the crankshaft is transmitted to that one of the camshafts which is positioned on the rear side of a vehicle. In addition, the rotational driving force transmitted to the rear side camshaft is further transmitted to the camshaft on the front side through a driving force transmitting mechanism between the camshafts.